An electron beam gun is a device which generates, accelerates, and focuses a beam of electrons. The elements of an electron gun can be divided into two broad categories: (1) the elements needed to generate free electrons, i.e., the cathode elements, and (2) the elements needed to shape, focus, and direct the electrons into a useful beam.
A primary use of electron beam guns is in the field of materials processing. Such guns can be used to evaporate or melt metals, alloys, and compounds. They can also be used to refine materials in order to produce very pure and strong materials, such as superalloys. Another common use of such guns is as a means of performing surface heat treatments of materials. With a sufficiently small beam size (on the order of 1 mm.sup.2 or less), the beam produced by such guns can also be used for cutting, welding, or drilling operations.
Standard electron beam guns consist of a cathode or emitter, which provides a source of electrons, and cathode and anode electrodes which produce an electric field between them for the purpose of accelerating the electrons produced by the emitter and guiding them into a transport or deflection system. The cathode and anode electrodes are usually separated and held in place by means of insulators, which provide structural rigidity and prevent a breakdown of the electric field between the electrodes.
The transport or deflection system typically consists of one or more assemblies which produce magnetic fields and are used to focus (converge) and direct the electron beam. The focused beam exits the electron beam gun and enters a magnetic transport system which directs it to the intended target. The interior of the gun, or the gun and target themselves may be contained within a vacuum in order to prevent beam divergence or loss of power due to interactions with the ambient environment.
The cathode, which is the source of electrons, can be a current carrying filament, a solid block which is indirectly heated by radiation from a filament, or it may be a material which is heated by an electron beam produced by another source. Cathodes can be characterized by their work function, which is the amount of energy (typically expressed in electron volts) required to overcome the potential barrier at their surface and result in the liberation of electrons.
In the case of a metal cathode, if such a cathode is heated sufficiently, some of the conduction electrons in the metal acquire enough energy to overcome the surface potential barrier and can be drawn off by a suitable electric field. If the field is high enough so that it draws off all the available electrons from a cathode of work function .PSI., the saturation current density J (in amps per cm.sup.2) produced at temperature T, is given by the Richardson-Dushman equation: EQU J=A.sub.o T.sup.2 exp (-e.PSI./kT), (1)
where A.sub.o is a constant whose value depends on the cathode material and usually has a value of 120 amps per cm.sup.2 deg.sup.2 e, is the charge on an electron and k is Boltzmann's constant.
There are several standard designs for electron guns, with the specific type used depending somewhat on the intended application. One of the most common designs is termed a "Pierce" gun which is designed to be operated with a space-charge limited cathode, i.e., operation of the emitter at less than the maximum value of the theoretically possible saturation current density. This mode of operation has the advantage that a smaller, virtual cathode is formed slightly in front of the physical cathode, with the virtual cathode having a stable and uniform electron current density which is independent of cathode temperature. A Pierce type gun also produces a uniform current density over the beam cross-section, and is capable of operating at a high level of efficiency.
During operation of an electron beam gun, it is important that the gun cathode, anode, and deflection system be properly aligned. This is because misalignment of these components can lead to a decline in the gun's performance. Such a decline in performance may be exhibited as a reduction in the beam quality, shape, or current (which reduces the power contained in the beam). The effects of misalignment are also observed in problems with beam steering and the potential for excessive heating of surrounding components of the gun. In existing electron gun designs, proper alignment is accomplished by means of adjusting screws and spacers which alter the relative positions of the gun's components.
The need for maintaining proper alignment illustrates a disadvantage of many existing, commercially available electron guns. Such guns support the cathode element from behind by means of a combination of joints, welds, flanges, and other mechanically coupled structures. During operation of the electron gun, the thermal expansion and distortion of these components can cause the cathode to become misaligned relative to the anode and downstream electron optics. Thus, present designs for commercially available electron guns are subject to problems associated with the use of adjustment screws and spacers, i.e., difficulties in achieving proper alignment and down time while the gun is out of operation. Present designs also require many parts and assembly steps which can lead to a large accumulation of error in the final product, as well as inconsistencies from gun to gun, all of which can affect gun system performance. Minimizing the number of components has the additional benefit of reducing the amount of parts stocking required.
Another problem common to many commercially available electron guns concerns the ease with which the electron source, i.e., the cathode or emitter may be replaced. Emitter failure is a common limiting factor on the lifetime of electron guns, and is usually dealt with by replacing the entire emitter assembly. In most existing guns, replacement of the emitter requires disassembly of the gun and the subsequent realignment of the emitter so that the gun's performance is not degraded. This can be time consuming and can impact the goals of the program for which the gun is being used. In some cases it may also be necessary or desirable to modify the emitter design, or to completely redesign the emitter assembly to achieve improved gun operation. This can be a problem in existing guns because the emitter assembly is an integral part of the cathode structure, so that it is difficult to modify its design without having an impact on the overall design of the cathode structure.
Yet still another problem with some designs for electron beam guns arises because the "triple point" where the insulators, electrodes, and vacuum meet can be the site of a breakdown in the high voltage electric field between the electrodes. Such a breakdown is capable of disrupting the operation of the gun and is usually prevented by spacing the electrodes a sufficient distance apart. This impacts the size, weight, bulkiness, and cost of the gun, which may affect its usefulness and performance, as well as limit future applications.
What is desired is a compact electron gun which has greater thermal and mechanical stability than presently available guns, a design such that the cathode is easier to align with respect to the anode and downstream optics, and in which the emitter or electron source can easily be replaced.